Secondary cells using a rechargeable hydrogen storage negative electrode are known in the art. These cells operate in a different manner than lead-acid, nickel-cadmium or other prior art battery systems. The hydrogen storage electrochemical cell utilizes a negative electrode that is capable of reversibly electrochemically storing hydrogen. In one exemplification the cell employs a positive electrode of nickel hydroxide material, although other positive electrode materials may be used. The negative and positive electrodes are spaced apart in an alkaline electrolyte, and may include a suitable separator, spacer, or membrane therebetween.
Upon application of an electrical current to the negative electrode, the negative electrode material (M) is charged by the absorption of hydrogen: EQU M+H.sub.2 O+e-.fwdarw.M--H+OH- (Charge)
Upon discharge, the stored hydrogen is released to provide an electric current: EQU M--H+OH-.fwdarw.M+H.sub.2 O+e- (Discharge)
The reactions are reversible.
The reactions that take place at the positive electrode are also reversible. For example, the reactions at a conventional nickel hydroxide positive electrode as utilized in a hydrogen rechargeable secondary cell or battery are: EQU Ni(OH).sub.2 +OH-.fwdarw.NiOOH+H.sub.2 O+e- (Charge), and EQU NiOOH+H.sub.2 O+e-.fwdarw.Ni(OH).sub.2 +OH- (Discharge).
A cell utilizing an electrochemically rechargeable hydrogen storage negative electrode offers important advantages over conventional secondary batteries. Rechargeable hydrogen storage negative electrodes offer significantly higher specific charge capacities (ampere hours per unit mass and ampere hours per unit volume) than do either lead negative electrodes or cadmium negative electrodes. As a result of the higher specific charge capacities a higher energy density (in watt hours per unit mass or watt hours per unit volume) is possible with hydrogen storage batteries than with the prior art conventional systems, making hydrogen storage cells particularly suitable for many commercial applications.
Suitable active materials for the negative electrode are disclosed in commonly assigned U.S. Pat. No. 4,551,400 to Sapru, Hong, Fetcenko and Venkatesan for HYDROGEN STORAGE MATERIALS AND METHODS OF SIZING AND PREPARING THE SAME FOR ELECTROCHEMICAL APPLICATION incorporated herein by reference. The materials described therein store hydrogen by reversibly forming hydrides. All the materials used in the '400 Patent utilize a generic Ti--V--Ni composition, where at least Ti, V, and Ni are present and may be modified with Cr, Zr, and Al. The materials of the '400 Patent are multiphase materials, which may contain, but are not limited to, one or more phases with C.sub.14 and C.sub.15 type crystal structures.
Other Ti--V--Zr--Ni alloys are also used for rechargeable hydrogen storage negative electrodes. One such family of materials are those described in U.S. Pat. No. 4,728,586 ("the '586 Patent") to Venkatesan, Reichman, and Fetcenko, the disclosure of which is incorporated by reference. The '586 Patent describes a specific sub-class of these Ti--V--Ni--Zr alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr. The '586 Patent, mentions the possibility of additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components of the alloys, and generally discusses specific additives and modifiers, the amounts and interactions of these modifiers, and the particular benefits that could be expected from them.
In contrast to the Ovonic alloys described above, the older alloys were generally considered "ordered" materials that had different chemistry, microstructure, and electrochemical characteristics. The performance of the early ordered materials was poor, but in the early 1980's, as the degree of modification increased (that is as the number and amount of elemental modifiers increased), their performance began to improve significantly. This is due as much to the disorder contributed by the modifiers as it is to their electrical and chemical properties. This evolution of alloys from a specific class of "ordered" materials to the current multicomponent, multiphase "disordered" alloys is shown in the following patents: (i) U.S. Pat. No. 3,874,928; (ii) U.S. Pat. No. 4,214,043; (iii) U.S. Pat. No. 4,107,395; (iv) U.S. Pat. No. 4,107,405; (v) U.S. Pat. No. 4,112,199; (vi) U.S. Pat. No. 4,125,688 (vii) U.S. Pat. No. 4,214,043; (viii) U.S. Pat. No. 4,216,274; (ix) U.S. Pat. No. 4,487,817; (x) U.S. Pat. No. 4,605,603; (xii) U.S. Pat. No. 4,696,873; and (xiii) U.S. Pat. No. 4,699,856. (These references are discussed extensively in U.S. Pat. No. 5,096,667 and this discussion is specifically incorporated by reference).
Still other suitable alloys for rechargeable metal hydride negative electrodes are described in U.S. Pat. No. 5,536,591, the disclosure of which is incorporated by reference herein.
The hydrogen storage negative electrode alloy is formed from a melt. The production of hydrogen storage negative electrodes utilizing the preferred materials is difficult because these preferred materials are not only not ductile, but are in fact, of relatively great or high hardness. Indeed, these alloys can typically exhibit Rockwell "C" hardnesses of 45 to 60 or more. Moreover, in order to attain high surface areas per unit volume and per unit mass, the alloy must be in the form of small ash or flake-like particles. In a preferred exemplification, the hydrogen storage alloy powder must pass through a 200 U.S. mesh screen, and thus be smaller than 75 microns in size (200 U.S. mesh screen has interstices of about 75 microns). Therefore, the resulting hydrogen storage alloy material must be comminuted, e.g., crushed ground, milled, or the like, to form a powder. The powder is then applied to an electrically conductive substrate, such as a wire mesh, wire screen, or expanded metal, to form a negative electrode. Preferably, the active electrode powder is compressed onto the substrate by a compaction apparatus such as a rolling mill. In an electrode fabrication process, it is important that the active electrode powder be uniformly applied to the conductive substrate. Variations in powder delivery results in a nonuniform density of active material in the electrode causing inadequate electrode and battery performance.
A method and apparatus for fabricating negative electrodes is disclosed in commonly assigned background art U.S. Pat. Nos. 4,820,481 and 4,915,898. The disclosures of U.S. Pat. Nos. 4,820,481 and 4,915,898 are incorporated herein by reference. Both patents describe an electrode fabrication process wherein active electrode powder is applied to and subsequently compressed onto an electrode substrate.
There exists a need in the art for an improved powder delivery system for an electrode manufacturing process which can deliver the active electrode powder at a constant rate and ensure a uniform and controlled application of the active material to the substrate. The powder delivery system disclosed herein is an improvement over that disclosed in the aforementioned background art and can provide for a more uniform delivery of electrode powder.